The present invention relates to a device for producing a scoring line in the cylindrical wall of a plastic cup and to a machine which uses said device. The invention is used in particular in so-called tamper-evident screw caps composed of a cup and of a tamper-evident ring which protrudes from the rim of the cup and in which the scoring line between the wall of the cup and the tamper-evident ring allows to separate the cup from the tamper-evident ring when the cap is unscrewed from the container to which it has been applied.
Machines using devices for producing a scoring line between the cup and the tamper-evident ring in screw caps for closing containers are already known. These devices essentially consist of a circular cutter against which the cap is made to roll so as to produce a scoring line which causes the tamper-evident ring to break away from the cup when the cap is unscrewed from the container. In order to achieve rolling of the cap, the cap is placed over a mandrel which follows a path which is concentrically external to the circular cutter and presses the wall of the cap against the sharp edge of the cutter. Such devices are disclosed for example in EP-533 633 and in U.S. Pat. Nos. 3,824,941, 3,861,551, 4,343,754 and 4,904,435.
A device with the features of the preamble of claim 1 is known from the document DE-A-1 482 603.
Conventional devices entail some substantial drawbacks which can be substantially ascribed to the fact that they are highly sensitive to variations in the external and internal shape of the cup. Moreover, it often happens that the cut is not performed perfectly due to the fact that the cap can slip on the mandrel and thus may not complete its rotation over the cutter.
The aim of the present invention is to provide a device which allows to obviate the above-mentioned drawbacks of conventional devices.
This aim is achieved with a device for producing a scoring line in the cylindrical wall of a plastic cup, in accordance with the present invention, which has the features set forth in claim 1.